I don't know how much you are being sued for, but since it is enough to provoke your request for assistance, it is probably worth consulting a lawyer about. It sounds as though you got a cash advance by means of a teaser check sent along with a credit card statement. These loans usually have high interest rates and fees attached. Whoever is suing you will have to prove the existence of the contract, its terms, and the assignment from the original lender to the buyer. Often, debt buyers don't acquire the documentation that they will need to prove their case if the consumer answers the complaint properly in court. Find an experienced local consumer lawyer through the National Association of Consumer Advocates, www.naca.net.
Disclaimer: This site exists to provide information only. It is not legal advice. Answering your question does not create an attorney-client relationship. I am a Massachusetts lawyer. Any information provided on this site does not, except as explicitly stated, imply familiarity with laws or procedures peculiar to your state which may differ from those where I practice.